1. Field of the Invention
The present invention relates to electronic packaging. More particularly, the present invention relates to electronic packaging using flexible flip chip carriers with land grid array attachment arrangements for stiffening same.
2. Background and Related Art
There exists in the prior art a variety of ways of connecting integrated circuit chips to printed circuit boards (PCBs). One prior art approach is to connect the integrated circuit chip directly to the laminate PCB. However, one of the difficulties with such connections arises from the large difference in thermal coefficient of expansion (TCE) between the silicon chip and the PCB. This difference creates problems affecting reliability over a large number of thermal cycles. One approach to addressing the problems caused by differences in TCE between chip and PCB is to use some form of intermediate chip carrier. Such carriers offer several advantages including the fact that they may be either single chip carriers or multichip carriers. Typically, they are made of plastic, ceramic or flexible tape-like material. However, chip carriers made of plastic, ceramic or flexible material also have limitations in regard to overall reliability, cost and ease of assembly. Intermediate structures known as interposers, are also employed in a variety of design configurations to connect chips or chip carriers to laminate cards or PCBs. Typically, these interposers are designed to offer some additional form of compliance or flexibility in connecting chips or chip carriers to cards or PCBs so as to avoid stress, fracturing and the like caused by differences in TCE between chip or chip carrier and card or PCB.
Many advances are being made in organic packaging materials, such as, polytetrofluoroethylene (PTFE) based materials. Such materials, when used as flexible chip carriers, offer a low dielectric constant that improves electrical performance beyond that which is possible with standard FR-4 laminates, for example. Such materials also make possible improvements in wiring density. As chip density and machine speeds increase, advanced laminate materials for chip carriers become more important. In addition to reliance upon material advances, use of improved interconnect technologies, such as land grid array (LGA) interconnects, become more important.
The LGA interconnects rely on good surface contact between mating pads for reliable electrical connection. Accordingly, a constant and uniform compressive force must be maintained between the mating pads throughout the life of the electronic device in order to maintain reliable electrical inter-connection. In addition, air must be excluded to prevent oxidation of the contact surfaces where noble metals are not used.
A primary requirement for LGA connectors is rigidity of the mating arrays. Thin packages, such as tape ball grid array (TBGA)-type packages and ones that use materials with low elastic moduli, such as PTFE or unfilled build-up layers, must have stiffeners attached in order to prevent excessive bending. Typical of such arrangements is the peripheral stiffener described by Tsukamoto in U.S. Pat. No. 5,841,194. However, even with peripheral stiffeners, force is transmitted through the package and produces a small amount of localized bending in the chip carrier substrate.
In addition to localized bending, plastic flow of the dielectric materials, especially when a laminate such as PTFE is used, occurs over time with an applied load, particularly at elevated temperatures. This acts to increase deformation. The compressive load on an LGA connection can cause relaxation of the laminate and a decrease in the contact force between mating pads over time thereby leading to intermittent or constant xe2x80x9copenxe2x80x9d connections.
Another problem that arises from this localized bending or deformation is the fracture of circuit lines or cracking of solder masks. Such fracture or cracking occurs because neither of these structures can deform plastically to the same degree as PTFE or an unfilled polymer, for example, can deform.
Accordingly, it is an object of the present invention to provide an improved electronic package and method of making same.
It is a further object of the present invention to provide improved LGA connections between chip carrier and carrier substrate to thereby increase reliability.
It is yet a further object of the present invention to provide an improved chip carrier structure that allows maintaining a constant and uniform contact force between mating pads of an LGA connection.
It is still yet a further object of the present invention to provide an improved chip carrier structure and method of making same for use in LGA connection between chip and card or PCB, which structure restricts localized bending and plastic flow.
It is another object of the present invention to provide an LGA stiffener for use with a flexible chip carrier, and method for making same.
It is yet another object of the present invention to provide a thin compliant chip carrier with stiffeners arranged to prevent excessive localized bending and plastic flow so as to maintain a constant and uniform compressive force between mating pads in an LGA-type connection.
In accordance with the present invention, a thin, compliant chip carrier structure, such as PTFE-based structures, is provided with pads at one surface, matching the pads of a flip-chip, and LGA pads at the other surface, matching the LGA pads of a relatively rigid mating card or PCB. The compliant chip carrier is provided with various forms of LGA stiffeners and methods for making same. The stiffeners act to prevent excessive localized bending and plastic flow in the chip carrier that cause fractures, warping and cracking in circuit lines and connection interfaces. The stiffeners are such as to readily accommodate compliant LGA conductors or bumps.
These foregoing and other objects, features and advantages of the invention will be apparent from the following more particular description of a preferred embodiment of the invention, as illustrated in the accompanying drawings, wherein like reference members represent like parts of the invention.